Optical navigation devices use a light source to illuminate a navigation surface so that an optical imager can generate digital images for computing motion. For example, an optical imager can image a finger on a surface and generate a navigation signal based on comparisons of sequential images. However, as the size of conventional navigation devices is reduced, the physical space for the optical elements and sensors for detecting the optical navigation input are also reduced.
For example, some conventional optical navigation device packages have a thickness, or optical height, of about 2.5 mm. The optical height refers to the distance from an optical finger interface surface to a corresponding sensor. The optical height is also referred to as the optical track. An optical track, or thickness, of 2.5 mm is considered too thick for some implementations of handheld devices such as cellular telephones and small portable personal computing (PC) peripherals.
As the optical track of a device is reduced, the viewing angle of the optical sensor through an intermediate optical lens is affected. In particular, the reduced optical track increases the image intensity roll-off, which is the reduction in light at the edges of the optical lens. In many instances, the image roll-off increases exponentially relative to the decreased optical track. As one example, the image intensity at the edge of the optical lens may be about 40% of the image intensity at the center of the optical lens. This has an adverse effect of reducing the accuracy of the imaging lens. In particular, this problem of increased image intensity roll-off is highly affected when the total distance of the optical track is less than about twice the width of the optical sensor.